1. Field of the Invention
The present invention relates to a non-asbestos gasket such as a metal gasket which is to be used in, for example, a connecting portion between flange pipes, and that between a fluid apparatus such as a valve and a pipe.
2. Explanation of Related Art
Usually, a conventional gasket is produced by using asbestos as disclosed in Japanese Patent Application Laying-Open No. 58-15088. However, because of hazardous properties of asbestos, conventional gaskets made of asbestos are gradually converted to those made of other materials. Also in a usual application of pipes, substitution of an asbestos joint sheet gasket is further promoted. As a gasket material which is a substitute for asbestos, recently, a metal and the like attract attention.
A metal gasket is used in an environment of very high pressure and temperature, and hence the fastening pressure (or the minimum load surface pressure) required in sealing is high, or namely a large fastening force is necessary. Therefore, means for fastening and fixing a metal gasket by using plural bolts is usually employed. Depending on the section shape of a sealing portion, metal gaskets are classified into various types including a flat type, a corrugated type, a saw tooth type, etc. In a piping system for a fluid such as liquid, a concentric circle-like and corrugated metal gasket is often used. As a metal gasket having a reduced number of corrugations, for example, a gasket which is to be interposed between a cylinder in an engine and a cylinder head is known as disclosed in Japanese Patent Application Laying-Open No. 2005-90699.
In a metal gasket having a corrugated section (also called “corrugated gasket”), a thin metal plate is formed into a concentric circle-like corrugated shape to obtain a high contact pressure, and the spring effect of the metal is added to improve the sealing properties. Such a metal gasket has features that, although made of a metal, a sufficient sealing function is obtained at a relatively low fastening pressure, and that it has high heat resistance. Therefore, such a metal gasket is often used in a fluid apparatus such as a valve.
An example of a metal gasket having a large number of corrugations is a metal gasket g which is to be interposed between flange pipes as shown in FIG. 11 in order prevent a fluid from leaking. The metal gasket g shown in FIG. 11 is configured by a pressed steel plate, and has: a center hole 38 having an inner diameter which is equivalent to the diameter of internal flow paths w of flange pipes 31, 32; and a sealing portion 37 which has an annular shape having an outer diameter that does not interfere with mounting bolts 35, and in which the section is formed into a concentric circle-like (or substantially concentric circle-like) corrugated shape.
In a metal gasket for preventing a fluid from leaking, a measure for improving the sealing properties is taken by disposing a corrugated portion as described above. Since such a metal gasket is based on metal-to-metal contact, however, the fact is that the metal gasket is still inferior to an asbestos joint sheet gasket with respect to the performance of preventing a fluid from leaking, i.e., the sealing property. A non-asbestos gasket seems to have room for further improvement.